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The Milky Way’s Most Luminous Star Clusters: Engines of Galaxy Evolution

Mubdi Muhammad Rahman

Doctor of Philosophy 2012
Graduate Department of Astronomy and Astrophysics, University of Toronto

Massive young star clusters and OB associations (M > 10^4 Msolar) dominate the energetic feedback from stars into the interstellar medium. They contain the most massive and luminous stars in the Galaxy, which shape their environments through winds, ionizing flux, radiation pressure, and eventually supernovae, destroying their natal molecular clouds and inflating superbubbles. Few such clusters have been identified in our Galaxy. We systematically investigate the most luminous H II regions, which we identify using the WMAP foreground maps. We find that the 13 most luminous sources produce one-third of the Galaxy’s total ionizing luminosity, all with expected powering populations of M > 10^4 Msolar. These populations are grouped in small numbers of clusters or associations for each WMAP source. The emission from these regions is dominated by the diffuse component at large radii (0-70 pc) indicating a high leaking fraction of ionizing photons. Using 8 μm maps from Spitzer GLIMPSE and published radio recombination line observations, we resolve the large (> 1 degree) WMAP sources into 40 star forming complexes (SFCs) exhibiting shell morphology with evidence of expansion due to a central powering source. We develop a method, based on differential extinction of the galactic disk, to identify the SFC’s powering cluster candidates with 2MASS. We identify 22 cluster candidates within the 40 SFCs having extinctions consistent with their distances. With near-infrared spectroscopy from the New Technology Telescope, we have confirmed the existence of the most massive of these associations, the Dragonfish Association, with M = 10^5 Msolar. Of the 50 sampled stars, we identify 2 Luminous Blue Variable candidates, a Wolf-Rayet, and 15 O-type stars, consistent with the yield expected from the candidate contamination rate, verifying the candidate cluster identification method. This investigation doubles the number of massive young star clusters and OB associations known and produces the most complete picture of the upper end of the Galaxy’s cluster mass function to date.

Reproduced with permission. library@astro.utoronto.ca
July 23, 2012